1. Field of the Invention
One embodiment of the present invention relates to an organometallic complex, particularly, to an organometallic complex capable of converting triplet excitation energy into light emission. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each including the organometallic complex.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the present invention disclosed in this specification and the like relates to an object, a substance, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a power storage device, a memory device, an imaging device, a method of driving any of them, a method of manufacturing any of them, and the like.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In the basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage to this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that, for example, it can be formed to be thin and lightweight and has high response speed.
In the case of an organic EL element in which an EL layer containing an organometallic complex as a light-emitting substance is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property, and thus, a current flows. By recombination of the injected electrons and holes, the organometallic complex is raised to an excited state to provide light emission.
The excited state of the organometallic complex can be a singlet excited state (S1) or a triplet excited state (T1), and light emission from the singlet excited state is referred to as fluorescence and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S1:T1=1:3.
Among the organometallic complexes, a compound capable of converting singlet excitation energy into light emission is called a fluorescent compound (fluorescent material), and a compound capable of converting triplet excitation energy into light emission is called a phosphorescent compound (phosphorescent material).
The internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including a fluorescent material is thought to have a theoretical limit of 25%, on the basis of S1:T1=1:3, while the internal quantum efficiency of a light-emitting element including a phosphorescent material is thought to have a theoretical limit of 75%.
In other words, a light-emitting element including a phosphorescent material has higher emission efficiency than a light-emitting element including a fluorescent material. Therefore, a phosphorescent material capable of converting triplet excitation energy into light emission has been actively developed in recent years. An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention because of its high phosphorescence quantum yield (see Patent Documents 1 to 3, for example).